Check out this OP build. Not as high damage as the Berserk Armor build that Hayden's Studio hypothesized (or maybe a Samurai Armor build with tons of sword leveling via spell), but since this is ranged damage, it's feasible and reliable.
Scroll = +1 Mana
Mage Robe x20 = * 2^(20) Mana from scroll
Wizard Hat = 2^20 damage from scroll
Wasp = * 2 damage from scroll
Red Ring = + 2^20 damage from mana (first attack only)
Boomerang = * 2 damage from Red Ring
Total damage = 2^20 * 2 + 2^20 * 2 = 2^22 = 4,194,304
Wizard Hat is the core: then, all that's needed is even a single scroll, as many Mage Robes as possible, and food maintenance. The Robes give tons of mana via the scrolls, the Wizard Hat deals that mana as ranged damage, and the Philosopher's Stone (more than "Rich Mage, Poor Mage" with some books) collects that mana as gold. Tons of gold allows for removing no-longer needed Symbols, and tons of refreshing at the store to get the ideal mix of trinkets, and spamming Epic keys (buy, then delete, then refresh until it shows up again) to pick up more Mage Robes. Wasp gives a bonus doubling, although past 2^10, everything (except the one "Stubborn" foe) is a one-hit KO anyway. For the Stubborn foe, keep more scrolls at first.
Necromancer Hands is helpful for getting duplicate items, to get to this point. Goblin Hand in the beginning helps to get sufficient scrolls (and books, although ultimately they aren't needed) and Demon Hand later on helps get sufficient Watermelons and Mage Robes. Red Ring and Boomerang add an extra chunk of damage for the first hit only.
Contra Hayden's Studio's awesome analysis, I love the multiplicative scaling, and hope there's a way to keep it.
(Also note that a Berserk Armor build would have higher base damage, but still need food [two Watermelons plus Fruit Salad works] and some leftover space for enemies, so the max number of practically feasible Berserk Armor is, like the Mage Robes here, 20. But there would be a low probability on each turn of actually hitting.)